Auto merge of #14008 - upsuper:rwlock, r=SimonSapin

Make style::context use parking_lot::RwLock

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The main motivation is to use `parking_lot::RwLock` instead of `sync::RwLock` in `style::context`, so that we can make `Stylesheet` be hold in a `parking_lot::RwLock` like other structs used for geckolib.

r? @SimonSapin

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- [ ] These changes fix #__ (github issue number if applicable).

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